Materials for printing electrical circuits on electrical conductor substrates known as PARMOD® materials are disclosed in U.S. Pat. Nos. 5,882,722, 6,036,889, 6,143,356 and 6,379,745, the entire disclosures of which are expressly incorporated herein by reference. PARMOD® materials have been developed for printing conductive circuits on polymer or paper substrates such as those used for printed wiring boards, flexible circuits and RFID antennae. Typically, polymer thick film conducting materials are made of individual particles which may be in adventitious contact with each other. In contrast, using PARMOD® materials and a simple print-and-heat process for “chemical welding” of pure metals, electrical conductors made of a single-phase continuous well-bonded metal trace are produced. PARMOD® materials also provide a desirable alternative to the conventional thick film compositions that are cured at high temperatures onto ceramic or glass based substrates. PARMOD® materials are cured at temperatures which polymer and paper based substrates can withstand, and provide electrical conductivity comparable to that of the pure metal and greater than that of polymer thick films.
A significant problem that arises in manufacturing conductive circuits on polymer or paper substrates is inadequate adhesion of the metal coating on the substrates. Yet another difficulty is achieving adequate adhesion while maintaining the desired resistivity properties in the electronic circuit. In general, a separate adhesive layer applied to the substrate surface has been required for sufficient adhesion of PARMOD® materials to rigid printed circuits (see, e.g., U.S. Pat. No. 6,379,745). For example, polyimide films are first coated with various adhesive layers before copper and silver PARMOD® compositions are printed on the surface and thermally cured to create flexible printed circuits. Suitable substrates for this purpose include Kapton® type FN with a FEP Teflon® coating; Kapton® types KJ and LJ with low melting polyimide coatings; and polyimide substrates with polyamic acid coatings. Copper PARMOD® compositions have been printed on rigid polyimide-glass laminates coated with a chain extending polyimide adhesive and thermally cured to create rigid printed circuits (see U.S. Pat. Nos. 6,143,356 and 6,379,745). However, because the adhesive layer infiltrates into the porous metal trace during curing, the curing conditions are predominately dictated by the properties of the adhesive rather than the PARMOD® materials which can cure at lower temperatures and in shorter times than the adhesive. Thus, adding the adhesive coating diminishes the advantages provided by the PARMOD®method and compositions. In the case of circuits with drilled holes for through-hole components and vias for electrical connections between layers, coating the holes with adhesive makes it difficult to obtain good bonding to the metal traces. Even if adhesive coatings are selected, suitable adhesive coatings are not widely available on substrates of commercial interest, such as paper and polymer based substrates. In addition, coated substrates are generally more expensive than uncoated substrates. Therefore, although attempts have been made to improve adhesion of conductive coatings, a suitable solution to this problem has not heretofore been developed.
Thus, there is a need for methods and compositions that provide sufficient adhesion of PARMOD® compositions to substrates of interest, and which retain the highly conductive properties of the PARMOD® materials.